When Selecting Fibers for UHPC, How to Choose Between Metallic and Non-Metallic Fibers?

Fiber selection is a key element in UHPC material design, directly affecting the material's toughness and failure mode. Based on 28 years of R&D experience in fiber-reinforced concrete, Qinglong Group believes that the choice between metallic and non-metallic fibers requires comprehensive consideration of strength requirements, durability, cost factors, and application environment.

Metallic fibers, represented by copper-plated steel fibers, are the most commonly used reinforcing fibers in UHPC. Their tensile strength typically exceeds 2000MPa, with an elastic modulus of 200GPa, effectively improving the material's tensile strength and toughness. Qinglong Group gives priority to copper-plated steel fibers in UHPC for load-bearing structures, with a diameter of 0.2mm, a length of 13mm, and a volume fraction of 2.5%. In the Shanghai Century Plaza project, steel fiber-reinforced UHPC achieved a flexural strength of 25MPa, meeting the safety requirements of large public buildings.

Non-metallic fibers mainly include carbon fiber and basalt fiber. Carbon fiber offers excellent tensile strength (3000-5000MPa) and corrosion resistance, but its price is relatively high and its bond performance with the cement matrix requires special treatment. Qinglong Group uses carbon fiber in special projects requiring resistance to magnetic interference, where a volume fraction of just 1.5% achieves a reinforcing effect comparable to steel fiber.

Fiber geometric parameters have a significant impact on the reinforcing effect. The aspect ratio is an important parameter; the aspect ratio of steel fiber is usually controlled between 60 and 80 to ensure a good pull-out effect rather than fracture failure. Qinglong Group's test data shows that when the aspect ratio increases from 40 to 80, the fracture energy of UHPC can be improved by more than 50%. Fiber surface morphology also requires attention: hooked-end steel fibers provide better anchorage than straight fibers.